The present invention relates to a latex comprising water and a styrenic block copolymer and to a process for preparing articles therefrom. In particular, the present invention relates to a process for preparing articles from a latex that comprises a particular styrenic block copolymer and wherein a surface is coated with the latex to obtain a film from which a thin article with improved tensile strength in combination with enhanced comfort can be obtained.
Natural rubber latex concentrates have been used for the manufacturing of rubber-dipped articles, adhesives, rubber thread, foam rubber etc. Natural rubber latex is in particular used for the manufacture of dipped articles such as: household gloves, examination gloves, industrial gloves, surgical gloves, catheters, teats and soothers, breather bags, tubing, balloons and condoms. When these articles are made by dipping it means that a surface or mould is dipped into the latex, thereby obtaining a coating of the rubber on the surface or on the mould.
Synthetic latices based on isoprene rubber are a very convenient replacement for natural rubber, as they do not suffer from the various allergens found in natural rubber. On the other hand, the production of high quality isoprene rubber and latices thereof is not easy. It thus remains of interest to find a synthetic latex that is relatively easy to make, free of allergens, that may be used for the preparation of dipped articles, and gloves, catheters and condoms in particular, with an improved balance of properties.
For instance, from U.S. Pat. No. 3,360,599 the preparation of high tensile strength films is known, made by annealing latices of A-B-A type block copolymers. In these block copolymers A represents a non-elastomeric polymer block, e.g. from lower olefins or a mono-vinyl arene, and B is an elastomeric polymer block, e.g. from a conjugated diene. Block copolymers according to this reference exhibit what are referred to as “self-curing” properties. By this is meant that the block copolymer exhibits the properties normally met in an elastomer which has been cured with the usual vulcanizing agents such as sulfur-containing compounds. The absence of a vulcanizing agent in such polymers is of great utility for many purposes, such as in the formation of molded articles, films, coatings or impregnated articles or in the preparation of latices, paints or adhesives.
From the even earlier U.S. Pat. No. 3,238,173, it was known that synthetic rubbers of the type A-B-A are self-vulcanizing rubbers. Latices containing such rubbers were found useful, e.g. in the preparation of dipped goods.
The above two references clearly show that in the preparation of dipped articles A-B-A type block copolymers are typically used without vulcanization packages.
Moreover, it is important that the articles made of synthetic rubber have a sufficiently high strength. For instance, in WO 2007/017368 it is described that one difficulty in the field of gloves for example, is the production of thin elastomeric articles having a high tensile strength. The solution that was found in WO 2007/017368 was the use of a specific vulcanization package that ensured that dipped articles from isoprene rubbers could be obtained with satisfactory tensile strength. The synthetic rubber used in this reference is not a self-curing type polymer of the type A-B-A.
This was a major improvement since the requirements for unaged surgical gloves dipped from natural rubber and synthetic rubber latices are specified in ASTM D3577. The mechanical requirements are tensile strength, ultimate elongation and stress at 500% elongation (also referred to as 500% modulus) measured according to ASTM D412. These requirements are listed in the table 1 below.
TABLE 1requirements for surgical glovesUltimateStress at 500%Tensile strengthelongationelongationTypeminimum valueminimum valuemaximum valueI (natural rubber)24 MPa750%5.5 MPaII (synthetic17 MPa650%7.0 MParubber)
High-strength films prepared from aqueous dispersions of block copolymers of vinyl aromatic monomers and conjugated dienes are known from U.S. Pat. No. 5,563,204. It describes an aqueous dispersion comprising one or more block copolymers of the formula A-B-Xm-(B-A)n wherein each A polymer block consists essentially of a monovinylidene aromatic monomer, such as styrene, and each B block consists of a conjugated diene, such as isoprene. The blocks A have a weight average molecular weight from 8,000 to 15,000, each block B has a weight average molecular weight of 30,000 to 200,000, and the average A block content in the block copolymer may be 5 to 25% wt. These dispersions are capable of forming a free-standing, coherent, elastomeric, solid film which, after drying and annealing at 80° C. for 30 minutes, demonstrates a tensile strength of about 11.0 MPa or greater. These dispersions are stated to be suitable for use in the manufacture of surgical gloves, condoms, catheters, balloons and other thin elastomeric articles.
High strength, however, is not the only issue. Another factor that is as important in the manufacture of gloves, condoms and catheters, is comfort. Merely increasing the tensile strength is not sufficient if this causes discomfort to the users. The problem underlying the current invention is to increase strength whilst at least maintaining comfort.
In the requirements for surgical gloves according to ASTM D3577 the stress at 500% elongation is used to indicate stiffness and flexibility. However, it can hardly predict these comfort-related properties at low percentages of elongation during the actual use of the gloves. A much better parameter to indicate flexibility and softness is measurement of stress at very low values of elongation, such as the Young's modulus, being the modulus at zero percent elongation. Another factor is the thickness of the film which also affects the feeling of comfort during use. Evidently, the film thickness may be influenced by concentration of the rubber in the latex or the application methods, including number of times that the article is dipped and/or the duration of each dipping. Evidently, when an industrial process is being applied, it would be advantageous if thinner films could be prepared without unnecessary amendments to the process. It has now been found that if one goes against 50 years of practice in the preparation of dipped goods from a latex of block copolymers and one subjects a latex of specific block copolymers of vinyl aromatic monomers and conjugated dienes to vulcanisation, an article is obtained with improved tensile strength, which moreover has good or even enhanced comfort-related properties. It appears that thinner films may be obtained from such a latex.